Allow selection of one or more members in an ensemble dataset

[b8raoult]

This feature allows the selection of one or more members of an ensemble dataset:

open_dataset("path-to-zarr", number=1)

and

open_dataset("path-to-zarr", number=[1, 3, 5])

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📚 Documentation preview 📚: https://anemoi-datasets--156.org.readthedocs.build/en/156/

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.96%. Comparing base (2c1e6b1) to head (20f2571).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #156   +/-   ##
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  Coverage    72.96%   72.96%           
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  Files           10       10           
  Lines          825      825           
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  Hits           602      602           
  Misses         223      223           

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